New features of the freezing vortex matter in single crystals of YBa₂Cu₃O_7-δ

We report on the new features observed in the magnetization of a high-T_c superconductor at cooling through the first order transition. The single crystal of YBa₂Cu₃O_7-δ cooled in the magnetic field H ∥ c at cooling (FCC) exhibits a multipolar magnetic moment which results from the summing up over the sample of both the negative (magnetization of the uncompressed vortex lattice) and the positive local magnetic moments. The positive local moments correspond to the enhanced density of the trapped flux lines exceeding locally that for the external magnetic field. At large fields (> 3T) below freezing temperature T_f, the dipolar contribution into the magnetic moment is fully reversible, in agreement with zero density of critical current. The magnetization of the field-cooled sample measured at warming (FCW) exhibit jumps at clearly larger temperature T_m(> T_f). The jump from zero to a finite value in the octupolar magnetizations ΔM_FCC^oct, ΔM_FCW^oct occurs at the same temperatures T_f, T_m as the conventional jump in the field-cooled dipolar magnetization ΔM_FC^dip related to the vortex ordering entropy (ΔS < k_B). The high-pole magnetic moments are metastable and exhibit long sample-specific relaxation, quickest in a narrow temperature range at T < T_m close to T_m.